Positive dispersion optical waveguide

ABSTRACT

A single mode optical waveguide fiber having a positive total dispersion is disclosed. The novel waveguide fiber has a core region comprising three distinct segments. Studies of this novel waveguide, wherein properties are calculated as various ones of the core region parameters are changed, show that the waveguide satisfies the requirements of a fiber in a high bit rate, long regenerator spacing system. The novel waveguide design is relatively simple to manufacture and maintains its functional properties at tight tolerances when the core region parameters are varied over a prescribed range. This high performance waveguide limits self phase modulation and four wave mixing, facilitates wavelength division multiplexing, and is compatible with optical amplifiers.

BACKGROUND OF THE INVENTION

The invention is directed to a single mode optical waveguide fiberwherein the total dispersion is maintained positive over the entirefiber length. The dispersion sign convention generally used in the artis, the dispersion of a waveguide is denoted as positive, if, in thewaveguide, a shorter wavelength signal travels at higher speed than alonger wavelength signal.

Because of the high data rates and the need for long regeneratorspacing, the search for high performance optical waveguide fibersdesigned for long distance, high bit rate telecommunications hasintensified. An additional requirement is that the waveguide fiber becompatible with optical amplifiers, which typically show an optimum gaincurve in the wavelength range 1530 nm to 1570 nm.

In cases where waveguide information capacity is increased by means ofwavelength division multiplexing (WDM) technology, an additionalwaveguide fiber property becomes important. For WDM, high bit ratesystems, it is necessary for the waveguide to have exceptionally low,but non-zero, total dispersion, thereby limiting the non-lineardispersion effect of four wave mixing.

Another non-linear effect which can produce unacceptable dispersion insystems having a high power density, i.e., a high power per unit area,is self phase modulation. Self phase modulation may be controlled bydesigning a waveguide core which has a large effective area, therebyreducing the power density. An alternative approach is to control thesign of the total dispersion of the waveguide so that the totaldispersion of the waveguide serves to counteract the dispersive effectof self phase modulation.

A waveguide having a positive dispersion, where positive means shortwavelength signals travel at higher speed than those of longerwavelength, will produce a dispersion effect opposite that of self phasemodulation, thereby substantially eliminating self phase modulationdispersion.

Thus there is a need for an optical waveguide fiber which:

is single mode over the wavelength range 1530 nm to 1570 nm;

has a zero dispersion wavelength outside the range 1530 nm to 1570 nm;

has a small, positive total dispersion over the wavelength range 1530 nmto 1570 nm; and,

retains the usual high performance waveguide characteristics such ashigh strength, low attenuation and resistance to bend induced loss.

From the cost and process point of view, ease of manufacture andinsensitivity of waveguide properties to process variations are alsohighly desirable properties.

The concept of adding structure to the waveguide fiber core by means ofcore segments, having distinct profiles to provide flexibility inwaveguide fiber design, is described fully in U.S. Pat. No. 4 715,679,Bhagavatula. The segmented core concept can be used to achieve unusualcombinations of waveguide fiber properties, such as those describedherein.

DEFINITIONS

The following definitions are in accord with common usage in the art.

The terms refractive index profile and index profile are usedinterchangeably.

The radii of the regions of the core are defined in terms of the indexof refraction.

A particular region begins at the point where the refractive indexcharacteristic of that region begins and ends at the last point wherethe refractive index is characteristic of that region. Radius will havethis definition unless otherwise noted in the text.

The core preform diameter is the overall core region dimension prior todrawing the draw preform to the final waveguide fiber diameter. The termdoes not in any way limit the novel profile to a specific waveguidemanufacturing process or imply that a particular process is superior toothers in manufacturing a waveguide having the novel profile.

The initials WDM represent wavelength division multiplexing.

The initials SPM represent self phase modulation, a non-linear opticalphenomenon wherein a signal having a power density above a specificpower level will travel at a different speed in the waveguide relativeto a signal below that power density. SPM causes signal dispersioncomparable to that of linear dispersion having a negative sign.

The initials FWM represent four wave mixing, the phenomenon wherein twoor more signals in a waveguide interfere to produce signals of differentfrequencies.

The term, % Δ_(i), represents a relative measure of refractive indexdefined by the equation,

    % Δ.sub.i =100×(n.sub.i.sup.2 -n.sub.c.sup.2)/2n.sub.i.sup.2,

where n_(i) is the maximum refractive index in region i, unlessotherwise specified, and n_(c) i s the refractive index in the claddingregion

For the refractive index profiles disclosed and describes herein, thesecond core segment index Δ%, i.e., Δ₂ %, refers to the minimumrefractive index in that core region.

The term alpha profile refers to a refractive index profile, expressedin terms of % delta(r), which follows the equation,

    %delta(r)=%delta(r.sub.o,)(1- (r-r.sub.o)/(r.sub.1 -r.sub.o)!.sup.alpha),

where r is in the range r_(o) ≦r≦r₁, delta is defined above, and alphais an exponent which defines the profile shape.

The pin array bend test is used to compare relative resistance ofwaveguide fiber to bending. To perform this test, attenuation loss ismeasured for a waveguide fiber with essentially no induced bending loss.The waveguide fiber is then woven about the pin array and attenuationagain measured. The loss induced by bending is the difference betweenthe two measured attenuations. The pin array is a set of ten cylindricalpins arranged in a single row and held in a fixed vertical position on aflat surface. The pin spacing is 5 mm, center to center. The pindiameter is 0.67 mm. The waveguide fiber is caused to pass on oppositesides of adjacent pins as shown in FIG. 4. During testing, the waveguidefiber is placed under a tension just sufficient to make the waveguideconform to a portion of the periphery of the pins.

SUMMARY OF THE INVENTION

A family of waveguide fibers has been discovered which meet the highperformance requirements outlined above. The novel optical waveguidefiber is characterized by a segmented core, i.e., a core region whichhas at least two distinct segments, each segment having a particularrefractive index profile as described in U.S. patent application Ser.No. 08/323,795, now U.S. Pat. No. 5,483,612.

A first aspect of the novel waveguide fiber is characterized by a coreregion having three segments. Each segment is described in terms of itsmaximum refractive index, n_(i), i=1,2,3, its radius, r_(i), measuredfrom the longitudinal symmetry axis of the waveguide, and its refractiveindex difference, Δ_(i), defined relative to the clad index n_(c). Theradii are defined in the detailed description section of thisspecification.

In addition, each segment has a characteristic refractive index profile.The novel three segment core waveguide fiber has, for the first centralsegment of the core, an alpha profile, where alpha=1. The indexdifference, Δ₁ % is ≦0.90%. The second segment, is an annular ringadjacent to and surrounding the central segment. The annular ring issubstantially flat and has Δ₂ % ≦0.024%. The third core segmentsurrounds and is adjacent to the second segment. The third segment has aprofile in the shape of a rounded step. The refractive index differenceis Δ₃ % ≦0.20%. As can be seen from the values of Δ_(i) %, the maximumrefractive index n_(i) of each segment is set so that n₁ >n₃ >n₂≧n≧n_(c), where n_(c) is the clad refractive-index.

Given this three segment core having the profile shapes as described andthe Δ_(i) % stated, it is possible to find numerous sets of radii, r₁,r₂, and r₃, which provide a waveguide fiber having the properties:

zero dispersion wavelength, λ_(o), ≦1530 nm;

total dispersion slope<0.065 ps/nm² -km over the range 1530 nm to 1570nm;

pin array bend loss≧12 dB;

mode field diameter≦7.4 microns;

cut off wavelength (measured on the fiber), λ_(c) <1450 nm; and,

total dispersion which is positive and has a magnitude in the range ofabout 0.50 to 3.0 ps/nm-km over the wavelength range 1530 nm to 1570 nm.

The novel profiles have Δ's which may generally be described, Δ₁ % inthe range of about 0.57% to 0.90%, Δ₂ % in the range of about 0 to0.024%, Δ₃ % in the range of about 0.08% to 0.20. The radii r₁, r₂, andr₃ are in the respective ranges of about, 3.0 microns to 3.8 microns,5.7 microns to 12.05 microns, and 6.8 microns to 12.4 microns. Thenumber of profiles which conform to these limits on Δ% and radius islarge without bound. It will thus be understood that it is not possibleto investigate all of the profiles having parameters within theseranges. Also it will be understood that not all of the profiles, withinthe parameter ranges given, have the properties of the novel waveguide.However, sufficient modelling work has been done to prove that thesedisclosed ranges of profile parameters fairly describe the scope of theinvention, as can be seen from Table 1 below which gives some ranges ofwaveguide fiber parameters investigated.

In a preferred embodiment of this first aspect of the invention, theprofile parameters Δ₁ %, Δ₂ %, and Δ₃ % are about 0.73%, 0.012%, and0.18% respectively, and r₁, r₂, and r₃ are about, 3.4 microns, 9.0microns, and 9.6 microns respectively. These values may be regarded ascenter values of the profile parameters. That is, these profileparameters are an ideal starting point to begin investigating theallowed ranges of each parameter.

A second aspect of the invention is a subset of the first aspect. Thefamily of refractive index profiles which belong to this subset haveparticularly attractive properties as a high performance waveguide whichis relatively insensitive to manufacturing variations. The waveguidefiber of this second aspect has a three segment core refractive indexprofile. The index profile parameters ranges are defined as intervalsabout the center values defined in the preferred embodiment of the firstaspect of the invention. In particular, for the family of novelrefractive index profiles, Δ₁ % is 0.73% +/-0.05%, Δ₂ % is 0.12%+/-0.12%, Δ₃ % is 0.18% +/-10.05%, r₁ is 3.4 microns +/-0.4 microns, r₂is 9.0 microns +/-3.0 microns, r₃ is 9.6 microns +/-2.8 microns, and ris 10.2 microns +/-3 microns.

This family of index profiles provides a waveguide fiber having modefield diameter greater than 8.3 microns, a bend loss less than 8 dB, anda positive total dispersion in the range of about 0.5 ps/fnm-km to 3.0ps/nm-km, over the wavelength range 1530 nm to 1570 nm. Thus thewaveguides of this subset have properties superior to those of theparent set defined in the first aspect of the invention.

It will be understood that waveguide fibers having index profileparameters which lie in respective ranges about twice those statedimmediately above may have the properties of the subject novel waveguidefiber. Thus, the respective parameter limits given immediately above arebelieved to be conservative in terms of the number of sets of workablewaveguide parameter sets contained therein.

An additional feature of this second aspect of the invention is theinsensitivity of the waveguide fiber properties to variations inwaveguide fiber profile due to manufacturing variations. The propertiesof the waveguide fiber were modelled under the constraint that all butone of the profile parameters was held at its center position. Theremaining profile parameter was allowed to vary between its limits asdefined in the second aspect of the invention.

Thus when Δ₁ % was allowed to vary by +/-0.05%, the properties of thewaveguide fiber were calculated to be:

mode field diameter>8.3 microns;

pin array bend loss<7 dB;

λ_(c) in the range of about 1390 nm to 1410 nm;

λ_(o) in the range of about 1510 nm to 1515 nm; and,

dispersion slope in the range of about 0.059 ps/nm² -km to 0.061 ps/nm²--km, over a wavelength range 1530 nm to 1570 nm.

When core radius r was allowed to vary by +/-0.08 microns, theproperties of the waveguide fiber were calculated to be:

mode field diameter>8.3 microns;

pin array bend loss<8 dB;

λ_(c) in the range of about 1380 nm to 1450 nm;

λ_(o) in the range of about 1500 nm to 1525 nm; and,

dispersion slope in the range of about 0.059 ps/nm² --km to 0.061 ps/nm²--km, over a wavelength range 1530 nm to 1570 nm.

When Δ₃ % was allowed to vary by +/-0.05%, the properties of thewaveguide fiber were calculated to be:

mode field diameter>8.35 microns;

pin array bend loss<6 dB;

λ_(o) in the range of about 1250 nm to 1550 nm;

λ_(o) in the range of about 1500 nm to 1525 nm; and,

dispersion slope in the range of about 0.059 ps/nm² --km to 0.061 ps/nm²--km, over a wavelength range 1530 nm to 1570 nm.

When r₃ was allowed to vary by +/-0.25 microns, the properties of thewaveguide fiber were calculated to be:

mode field diameter>8.35 microns;

pin array bend loss<6 dB;

λ_(c) the range of about 1350 nm to 1450 nm;

λ_(o) in the range of about 1510 nm to 1513 nm; and,

dispersion slope in the range of about 0.059 ps/nm² --km to 0.061 ps/nm²--km, over a wavelength range 1530 nm to 1570 nm.

It is contemplated that favorable interactions among the parameters,i.e., interactions wherein movement of one parameter cancels thenegative result of movement of another parameter, will greatly increasethe tolerance limits on the family of waveguide fibers describedimmediately above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general representation of a three segment core refractiveindex profile which shows the definitions of the index profiledimensions.

FIG. 2 is an example of the novel refractive index profile of thisapplication.

FIGS. 3a, 3b, 3c, and 3d show the sensitivity of waveguide fiberproperties to changes in the refractive index profile parameters.

FIG. 4 is a schematic top view of the pin array bend test.

DETAILED DESCRIPTION OF THE INVENTION

The novel optical waveguide fiber described herein includes a coreregion having three segments. The segments are distinguished from eachother by a refractive index profile characteristic of a given segment.The three segment core region provides sufficient flexibility ofwaveguide fiber design to accommodate a wide range of functionalrequirements. The parameters which may be changed to provide particularwaveguide fiber performance are:

Δ% of each of the three regions;

radius of each of the three regions; and,

index profile shape of each of the three regions.

The distinguishing properties of the novel waveguide fiber disclosedherein are: positive total dispersion, over a prescribed wavelengthrange, 1530 nm to 1570 nm, to counteract the SPM non-linear effect; verylow dispersion slope over the prescribed wavelength range, to facilitateWDM operation; and, dispersion zero outside the prescribed wavelength tolimit dispersion due to four wave mixing. The positive dispersion istypically less than 3 ps/nm/km which enables long unregenerated systems.Advantageously, the prescribed wavelength range essentially coincideswith the peak gain curve of an erbium doped optical amplifier. Thus, thesubject novel waveguide fiber is uniquely suited for systems which carryhigh bit rate or incorporate optical amplifiers or long regeneratorspacing.

In addition, the core region design is simple, which means attenuationwill be comparable to that of step index fiber and the manufacturingcost is maintained as low as possible.

The excellent waveguide properties and performance include the samestrength and fatigue performance as step index waveguide fiber.Furthermore, the bend resistance of the subject novel waveguide fiber isas good as or better than that of dispersion shifted waveguide fiber nowavailable. The pin array bend test which confirms this statement ofrelative bend performance is shown in FIG. 4, which is a top view of thetest apparatus along with a fiber in position for testing. Waveguidefiber 32 passes on alternating sides of pins 34. The pins are fixedlymounted on a substrate 32. The fiber is tensioned such that the fiberconforms to the shape of a portion of the pin surface.

Referring to FIG. 1, the three core segments in which the profile can beadjusted are indicated as 2, 6, and 8. In each of the three segments,the index profile is defined by a particular refractive index at eachradial point of the segment. The radial extent of each segment may beadjusted to obtain preferred waveguide fiber properties. Asillustration, the radius of central core region 2 is shown as length 4.In this case, and for all modelled cases, the central core radius ismeasured from the axial centerline to the intersection of theextrapolated central profile with the x axis.

The first annular region 6 is delimited by the radius 4 and the radius7, which extends to vertical line 5 drawn from the half width point ofthe second annular region. The characteristic radius of the secondannular region 8 is chosen as length 12, which extends from the corecenter line to the midpoint of the base of segment 8, as indicated bypoint 3. This convention for second annulus radius is used in allmodelled cases. A convenient profile measure for symmetrical profiles isthe width 10 shown between vertical lines 5. Lines 5 depend from thehalf width points of segment 10. This convention for second annuluswidth is used in all modelled cases.

Example 1--Three Segment Positive Dispersion Waveguide Fiber

A representative of a three segment core region refractive index profileis shown in FIG. 2. The centerline dip in the central index profilesegment is due to diffusion of dopant from the waveguide fibercenterline during waveguide preform processing. The central segment isan alpha profile where alpha is about 1 and Δ₁ % is about 0.73%. Thecentral radius is about 3.4 microns. The second segment is annulus 18which has Δ₂ % near zero and inner and outer radii, 3.4 microns and 9microns respectively. The third segment, 20, has a width of about 0.95microns, a Δ₃ % of about 0.14%, and a radius to the midpoint of thesegment of about 9.5 microns.

The predicted performance of this waveguide is:

λ_(o) =1551 nm;

dispersion slope=0.06 ps/nm₂ --km;

mode field diameter=8.4 microns;

λ_(c) =1412 nm in fiber form and 1100 nm after cabling;

total dispersion in the range 1-3 ps/nm-km over the wavelength range1530 nm to 1570 nm; and,

pin array bend loss=5.6 dB, which compares favorably with the average 8dB loss of negative dispersion three segment waveguides.

Note that the waveguide fiber of example 1 meets every criteria of ahigh performance single mode waveguide fiber designed for WDM, limitedfour wave mixing, reduced SPM, and use with erbium doped opticalamplifiers.

The four charts, FIG. 3a, 3b, 3c, and 3d show the insensitivity of thenovel waveguide fiber to variations in the core region parameters.

Example 2--Bend Loss and Mode Field Sensitivity

FIG. 3a is a chart of bend loss vs. mode field diameter wherein Δ₁1%=0.73% is allowed to vary between limits +/-0.01Δ%. The radius of thecore preform prior to draw is allowed to vary by an amount of about2.5%, wherein the core preform radius prior to draw is generally in therange 3.5 mm-6 mm. This particular radius is chosen as a parameterbecause a variation in core preform radius can result in differentrelative spacing of the segments as well as differences in the segmentradii. For the third segment, Δ3% is taken to be 0.18%+/-0.05%. Thethird segment radius is 9.6 microns+/-0.25 microns.

To generate curves 22, 24, 26, and 28 of FIG. 3a, three of theparameters are held at their midpoint while the fourth parameter isvaried between its upper and lower limits. Thus, line 24, is found bycalculating bend loss and mode field diameter when preform radius is 3.5mm, Δ₃ % is 0.18%, r₃ is 9.6 microns, and Δ₁ varies over the range 0.72%to 0.74%. Likewise, line 22 is found by calculating bend loss and modefield diameter when Δ₁ % is 0.73%, Δ₃ % is 0.18, r₃ is 9.6 microns, andpreform radius varies over the range 3.5 microns +/-2.5%. Curves 26 and28 are generated analogously and specific parameter values are 0.18%+/-0.05% for Δ₃ % and for r₃, 9.6 microns +/-0.25 microns.

It is extraordinary that the core region parameters may be varied, asdescribed above, while bend loss remains below 8 dB and mode fielddiameter stays within the range 8.30 microns to 8.5 microns.

Table 1. shows the midpoint values of each of the core region indexparameters and the ranges which define the family of novel profiles.

                  TABLE 1                                                         ______________________________________                                                                                Segment                               Δ.sub.1 %                                                                         r.sub.1 Δ.sub.2 %                                                                        Δ.sub.3 %                                                                      r.sub.3                                                                             3 Width                               ______________________________________                                        Average                                                                             0.73%   3.4     0.012% 0.18%  9.6   1.2                                               microns               microns                                                                             microns                             Limits                                                                              +/      +/-0.2  +/.006%                                                                              +/-0.05%                                                                             +/-1.4                                                                              +/-0.5                                    0.10%                                                                   ______________________________________                                    

Example 2--Waveguide Cut Off and Mode Field Diameter

Referring to FIG. 3b, the four curves shown therein are generated in amanner analogous to the curves in FIG. 3a.

Note that for the stated variations in segment 1 delta, Δ₁ %, in preformradius, and in r₃, the cut off wavelength lies in the very narrow range1350 nm to 1450 nm. More variation in cut off is seen when preformradius varies over its prescribed range of about 3.5 mm+/-2.5%. Even inthis case, however, the fiber is fully functional because the cabled cutoff wavelength will be below about 1100 nm. In general, cabling causesthe cut off wavelength to decrease by about 400 nm relative to the cutoff wavelength measured for the waveguide fiber prior to any furtherprocessing.

The variation in mode field diameter is again confined to the narrowrange, 8.30 microns to 8.5 microns.

Example 3--Zero Dispersion Wavelength and Mode Field Diameter

As in examples, 1 and 2 above, each of the four core region parametersare allowed to vary over the selected range of values. Referring to FIG.3c, mode field diameter is in the range 8.3 microns to 8.5 microns andzero dispersion wavelength, λ_(o), is advantageously confined to therange of about 1500 nm to 1520 nm. Thus, for relatively large variationsin the parameters of the novel waveguide fiber core region, λ_(o)remains outside the WDM region which coincides with the peak gain rangeof an erbium doped optical amplifier, i.e., 1530 nm to 1570 nm.

Example 4--Total Dispersion Slope and Mode Field Diameter

As is shown in FIG. 3d, mode field is in the range 8.3 microns to 8.5microns and total dispersion slope lies in the narrow range 0.059 to0.061 ps/nm² --km, when the core parameters are taken through theirrespective ranges of variation.

Viewing the four charts, FIG. 3a, 3b, 3c, and 3d together theinsensitivity of the mode field diameter to the prescribed variation ofr₃ is remarkable. Also, the four parameters studied in these examplesare seen to have about equal impact on the variation in dispersionslope. The clustering of points in the sensitivity charts of FIG. 3strongly shows the ease of manufacture of the novel positive dispersioncore region design.

We anticipate even greater flexibility in terms of manufacturingtolerance on key core region parameters. We know that the parametersinteract and thus, we contemplate the effect of the variation of oneparameter cancelling the effect of the variation of another parameter.Thus, the study of the parameter variations in pairs or in sets of threeor more is contemplated, thereby defining a much broader family of coredesigns which yield positive dispersion over the key wavelength range aswell as those properties characteristic of a high performance waveguidefiber.

Although particular embodiments of the invention have hereinbefore beendisclosed and described, the scope of the invention is neverthelesslimited only by the following claims.

What is claimed is:
 1. A single mode optical waveguide fibercomprising:a core region having a refractive index profile includingthree segments, a first central segment having a maximum refractiveindex n₁, an index difference λ₁ %, and a radius a₁ measured from apoint on the longitudinal axis of symmetry of said waveguide fiber, saidcentral segment disposed substantially symmetrically about thelongitudinal axis of symmetry, a second segment, of annular shapedisposed adjacent and around said central segment, having a maximumrefractive index n₂, an index difference Δ₂ %, and an outside radius r₂,a third segment, of annular shape disposed adjacent and around saidsecond segment, having a maximum refractive index n₃, an indexdifference Δ₃ %, and a radius to the midpoint of said third segment r₃ ;a clad layer, of annular shape disposed adjacent and around said thirdsegment, having a maximum refractive index n_(c) ; wherein, n₁ >n₃ >n₂≧n_(c), said central segment has an alpha refractive index profile andalpha is about 1, said second segment has a substantially flatrefractive index profile, and said third segment has a rounded stepindex profile, Δ₁ %≦0.9%, Δ₂ % ≦0.024%, and Δ₃ % ≦0.2%, and the radii,r₁, r₂, and r₃ are chosen to provide an optical waveguide fibercharacterized by, λ_(o) ≦1530 nm, total dispersion slope<0.065 ps/nm₂--km over the range 1530 nm to 1570 nm, pin array bend loss <12 dB, modefield diameter ≦7.4 microns, λ_(c) <1450, and total dispersion which isin the range of about 0.5 ps/nm-km to 3 ps/nm-km over wavelength range1530 nm to 1570 nm.
 2. The single mode optical waveguide of claim 1wherein Δ₁ %, Δ₂ %, and Δ₃ % are in the respective ranges of about 0.57%to 0.90%, 0 to 0.24%, and 0.08% to 0.20%, and r₁, r₂, and r₃ are in therespective ranges of about, 3.0 to 3.8 microns, 5.7 to 12.05 microns,and 6.8 to 12.4 microns.
 3. The single mode optical waveguide of claim 1wherein Δ₁ %, Δ₂ %, and Δ₃ % are about 0.73%, 0.012%, and 0.18%respectively, and r₁, r₂, and r₃ are about, 3.4 microns, 9.0 microns,and 9.6 microns respectively.
 4. A single mode optical waveguide fibercomprising:a core region having a radius r, and a refractive indexprofile including three segments, a first central segment having amaximum refractive index n₁, an index difference Δ₁ %, and a radius a₁measured from a point on the longitudinal axis of symmetry of saidwaveguide fiber, said central segment disposed substantiallysymmetrically about the longitudinal axis of symmetry, a second segment,of annular shape disposed adjacent and around said central segment,having a maximum refractive index n₂, an index difference Δ₂ %, and anoutside radius r₂, a third segment, of annular shape disposed adjacentand around said second segment, having a maximum refractive index n₃, anindex difference Δ₃ %, and a radius to the midpoint of said thirdsegment r₃ ; a clad layer, of annular shape disposed adjacent and aroundsaid third segment, having a maximum refractive index n_(c) ; wherein,n₁ >n₃ >n₂ ≧n_(c), said central segment has an alpha refractive indexprofile and alpha is about 1, said second segment has a substantiallyflat refractive index profile, and said third segment has a rounded stepindex profile, and Δ₁ % is 0.73% +/-0.05%, Δ₂ % is 0.012% +/-0.012%, Δ₃% is 0.18% +/-0.05%, r₁ is 3.4 microns +/-0.4 microns, r₂ is 9.0 microns+/-3.0 microns, r₃ is 9.6 microns +/-2.8 microns, and r is 10.2 microns+/-3 microns, to provide a waveguide fiber having a mode field diametergreater than 8.3 microns, a bend loss less than 8 dB, and a positivetotal dispersion in the range of about 0.5 ps/nm-km to 3 ps/nm-km over awavelength range 1530 nm to 1570 nm.
 5. The single mode opticalwaveguide of claim 4 wherein the refractive index profile of said coreregion has parameters substantially centered on the respective toleranceintervals except Δ₁ % which has a value anywhere in its toleranceinterval, said optical waveguide has mode field diameter>8.30 microns,bend loss <7 dB, λ_(o) in the range of about 1390 nm to 1410 nm, λ_(o)in the range of about 1510 nm to 1515 nm, and a dispersion slope in therange of about 0.059 ps/nm² --km to 0.061 ps/nm² --km over a wavelengthrange 1530 nm to 1570 nm.
 6. The single mode optical waveguide of claim4 wherein the refractive index profile of said core region hasparameters substantially centered on the respective tolerance intervalsexcept r which has a value anywhere in its tolerance interval, saidoptical waveguide has mode field diameter>8.30 microns, bend loss <8 dB,λ_(c), in the range of about 1380 nm to 1450 nm, λ_(o) in the range ofabout 1500 nm to 1525 nm, and a dispersion slope in the range of about0.059 psfnm² --km to 0.061 ps/nm² --km over a wavelength range 1530 nmto 1570 nm.
 7. The single mode optical waveguide of claim 4 wherein therefractive index profile of said core region has parameterssubstantially centered on the respective tolerance intervals except Δ₃ %which has a value anywhere in its tolerance interval, said opticalwaveguide has mode field diameter>8.35 microns, bend loss<6 dB, λ_(c) inthe range of about 1250 nm to 1550 nm, λ_(o) in the range of about 1500nm to 1525 nm, and a dispersion slope in the range of about 0.059 ps/nm²--km to 0.061 ps/nm² --km over a wavelength range 1530 nm to 1570 nm. 8.The single mode optical waveguide of claim 4 wherein the refractiveindex profile of said core region has parameters substantially centeredon the respective tolerance intervals except r₃ which has a valueanywhere in its tolerance interval, said optical waveguide has modefield diameter>8.35 microns, bend loss <6 dB, λ_(c) in the range ofabout 1350 nm to 1450 nm, λ_(o) in the range of about 1510 nm to 1513nm, and a dispersion slope>0.059 ps/nm² --km and<0.061 ps/nm² --km overa wavelength range 1530 nm to 1570 nm.